Dish/Stirling Systems engines generate electricity by focusing solar energy with a parabolic dish concentrator onto the heat-input end of a Stirling engine. This arrangement is of interest because it provides the potential for high solar-to-electrical conversion efficiencies, and low costs can be achieved with mass production. Applications include utility scale generation, grid support, village electrification, remote power, end-of grid power and irrigation pumping.
Successful implementation of dish/Stirling systems requires hybrid operation (fossil-fuel co-firing) via reliable, efficient burner systems that can be installed at a capital cost increment competitive with conventional means for providing electricity from fossil fuels in a given market, e.g., diesel generator sets. Burner system lifetime is also a concern as are air emissions in many markets.
In most applications, a hybrid capability is needed to maintain operation at night or under cloudy conditions. In a hybrid installation, there is a thermal interface between the concentrated solar flux and the heat input end of the engine. The thermal interface can be a sodium heat pipe receiver which allows heat input from a burner as well as the parabolic solar dish concentrator. Metal matrix burners have received a great deal of attention lately because of their ability to bum fossil fuels with very low emissions of nitrogen oxides. In this type of burner, a premixed air/fuel stream flows through a porous metal matrix. A significant fraction of the heat of combustion is released as infrared radiation from the matrix. Because of this heat removal from the combustion zone, maximum temperatures are well below the adiabatic flame temperature resulting in emissions of nitrogen oxides (NOx) as low as 10 ppm at 0% oxygen without exhaust gas recirculation.
Matrix burners have typically been used in an outward burning arrangement in which air and fuel mixing takes place inside a cylinder-shaped matrix, and combustion occurs as the mixture flows radially outward through the matrix. In many applications, however, a cylindrical inward burning approach is needed. This approach has been avoided in the past because of the difficulty of obtaining uniform flow and uniform burning in the matrix.
The present invention teaches that flow distributors could be used and still maintain a low pressure drop. One problem is that there are a large number of possible configurations. Using a build-and-test approach, it would be expensive and time-consuming to test even a single configuration. Even after a proposed design is built, it would be very difficult to place instruments in the plenum, and so engineers could only learn the temperature and flow conditions at a few points. This would make it impossible to measure the flow patterns in the plenum, so engineers would be able to gain little understanding of why their design alternatives did not work.
The present invention uses Computational Fluid Dynamics, CFD, to evaluate the inlet plenum and flow distributor configurations. A CFD analysis solves the fundamental controlling equations and provides fluid velocity, species, pressure and temperature values at every point in the solution domain. This allows engineers to optimize fluid flow patterns or temperature distributions by adjusting either the system geometry or the boundary conditions, such as inlet velocity and temperature, wall heat flux, etc. Of particular benefit to this application is the fact that CFD enables detailed parametric studies that make it possible to optimize a design with relatively little time and expense.
FLUENT.TM. software from Fluent Inc. is used to perform the analysis. (Fluent, Inc., USA, 10 Cavendish Court, Centerra Resource Park, Lebanon, N.H., 03766-1442, Tel.: (603) 643-2600.) Modeling is done to determine if the preheated air/fuel mixture would pre-ignite (burn upstream of the burner matrix). This is important because high temperature applications require recuperation of exhaust gas energy via combustion air preheat to achieve good thermal efficiency.
The plenum forms a cylindrical annulus and distributes the air and fuel from four inlet pipes to the outside of the burner matrix. With the geometry imported, engineers create the mesh and run analysis iterations to model a uniformly-heated matrix which doesn't pre-ignite. In addition, uniform mixing of the air and fuel are critical. In constructing a prototype for an application specific dish/Stirling engine, a sodium heat pipe was necessary.
After solving the flow distribution problem, the inventors herein turned their attention to the primary heat exchanger. This device transfers heat from the combustion gases to the heat pipe. The design requirements were 75 kW heat transfer to the heat pipe, 5000 Pascal pressure drop and 1150.degree. K maximum metal temperature.
Analysis showed that strip fins provided inadequate heat transfer and would likely be subject to excessive thermal stress. Pin fins were found to provide adequate heat transfer, acceptable pressure drop and were not expected to suffer from significant thermal stress. The combustion gas flow begins in a radial direction along the pin axes and then turns across the pin axes. In addition, significant temperature change in the exhaust gas and resulting large property variations preclude the use of existing correlations. Finally, radiation heat transfer is an additional complicating factor that could be incorporated in the CFD work. Other applications such as air flowing through an open pipe might place pin fins inside the pipe, and might remove them from the outside of the pipe.
The next step was building a one/sixth scale prototype to validate the model. This system was designed and built in collaboration with engineers at Sandia National Laboratories, Albuquerque, N.Mex., and provided the capability to test all features of the burner, primary heat exchanger, and heat pipe. In this prototype system, only the heat pipe diameter was reduced to 1/6.sup.th full scale, all other dimensions were maintained at their full-scale values. The system was operated with the air/fuel mixture preheated up to 675.degree. C., thus validating the CFD calculations used to predict pre-ignition. Prototype test data showed very close correlation to the CFD results. Pressure drop across the primary heat exchanger and heat transfer through the primary heat exchanger agreed to within about 10% of the CFD results. Maximum pin tip temperature, agreed to within 10.degree. C. of the CFD model.
Summarized below are related patents in the art.
U.S. Pat. No. 5,749,720 (1998) to Fukuda et al. discloses a radially inward burning matrix burner which discharges the exhaust fumes into the central heat pipe. The collar-shaped intake manifold allow the fuel/air mixture to cool the matrix support clips to prevent pre-ignition. A single intake port supplies the fuel air mixture for the entire circumference of the ring-shaped matrix. No pre-heating of the fuel/air mixture is possible due to the unaddressed problems of pre-ignition which pre-heating creates. No suggestion of how to provide uniform circumferential burning is made wherein only a single inlet port is shown. Non-uniform burning results in burner matrix hot spots which forces an average lower burner throughput to prevent matrix damage. Thus, Fukuda et al. teaches an inefficient, oversized gas burner.
U.S. Pat. No. 5,667,374 (1997) to Nutcher et al. discloses an in-line (not radial) gas burner having a ceramic honeycomb with a plurality of axial passageways to form a planar flame face offering low NOx levels. An in-line plenum has a bluff body to thoroughly mix the fuel/air mixture before combustion in the honeycomb. A mesh flame trap is used to prevent pre-ignition. There are no suggestions of pre-heating the fuel.
U.S. Pat. No. 5,711,661 (1998) to Kushch et al. discloses a multi-layered matrix burner having at least three porous layers creating a three-dimensional array with the flame at the downstream porous layers. High-intensity applications are taught in the range of 1,500,000 BTU/hr/ft..sup.2 with large turndown ratios.
U.S. Pat. No. 5,603,905 (1997) to Bartz et al. discloses a hazardous waste combustion burner having a box-like construction of foraminous surfaces and a cooling column below.
U.S. Pat. No. 5,496,171 (1996) to Ozawa et al. discloses a round matrix burner having a flat, round burn surface which has a central burn area having a separate plenum feeding a lean fuel mixture. A second peripheral plenum has a rich fuel mixture which prevents a "lift" phenomenon of the flames. NOx emissions are reduced.
U.S. Pat. No. 5,205,731 (1993) to Reutter el al. discloses an axial flow nested fiber matrix burner which results in blue flame operation with low NOx levels comparable to radiant mode burners but with eight to ten times higher port loadings.
U.S. Pat. No. 5,211,552 (1993) to Krill et al. discloses a radially inward burning matrix burner in FIGS. 3, 4, 6, and 7. A refractory body reradiates heat from inside the combustion cavity to the matrix combustion surface. The combustion cavity acts as an exhaust port to exit the heat and exhaust gases. The combustion cavity must be devoid of a heat exchange means or used only to preheat air. The excess air in the range of 50-150% in excess of the stoichiometric requirement creates an invisible flame and a product gas of not more than 10 ppm NOx, 30 ppm CO and 10 ppm UHC. The embodiment of FIG. 6 is a square version of a radial burner having an intake manifold with four equally spaced inlet ports. No plenum design nor uniform flow measures are taught.
This invention only teaches a burner that develops useable heat only in the form of a super-heated exhaust gas created from more than 50% excess air while maintaining a clean burner. The burner pre-heats some of the air in the fuel/air mixture. Excess air is required in all matrix burners, otherwise the matrix will run too hot and burn out. All contemporary burners use excess air for this reason and also to reduce emissions.
U.S. Pat. No. 5,476,375 (1995) to Khinkis et al. discloses an in-line two-stage matrix burner which introduces a second oxidant into the porous bed.
U.S. Pat. No. 5,522,723 (1996) to Durst et al. discloses a combustion chamber having a porous matrix with pore-size increases from the inlet to the outlet to create a multi-zone burner.
The closest known prior art is Krill et al. shown herein as FIG. 2. FIG. 2 forms a furnace 40 useful for the practice of Krill's invention by having four surface combustors 30 arranged to provide a square adiabatic zone 41. Where each pair of combustors 30 meet at right angles to one another, a refractory post 42 is cemented to the side walls 32 of the contiguous burners 30 so that the products of combustion or flue gas cannot leak along the vertical (normal to FIG. 2) juncture line 43 of contiguous burners 30. By this arrangement, the four burners 30 act as an inwardly fired furnace. In short, FIG. 2 demonstrates that a furnace may be formed of modular burners 30. Likewise, preferably furnace 40 has a square refractory core 44 set in its center so that infrared radiation from the four porous fiber layers 35 will impinge thereon. Optional core 44 serves as a refractory column. It is understood that furnace 40 will have a base slab and a top slab with a stack opening.
Metal pan 31 has side walls 32 with screen 33 welded to the ends 34 of side walls 32. A porous layer 35 of ceramic fibers is deposited on, and attached to, screen 33. The porous layer 35 provides the exposed surface at which a mixture of fuel gas and air will burn without visible flame and become radiant. The fuel gas-air mixture is fed to combustor 30 through pipe 36 connected to metal pan 31.
The Krill device cannot produce uniform heat around center point E because of the varying distances from E to the corners and sides of the furnace. Also, the maximum burn temperatures cannot be achieved for the matrix 35 without creating hot spots at corners A, B, C, D due to the radiation view factor between surfaces near the corners. Therefore, Krill's furnace 40 fails to achieve a maximum heat output within his given geometry, thus using materials in construction and complicating construction costs with leak-prone corners A, B, C, D. It is estimated the Krill's device would only be half as efficient in heat out per area of burner as compared to the present invention. Krill does not teach a solution to uniform flow through the plenum with his preheated air/fuel mixture. Therefore, based on experimental modeling and successful prototyping used to develop the present invention, Krill will develop hot spots at the matrix centers at points F, G, H, I. Problems with these hot spots and those at the corners of the rectangular plenum will force Krill's furnace to run at less than maximum temperatures and throughput. Another drawback to Krill's furnace is the lack of heat exchange means in the furnace except for preheated fuel/air mixture. Krill's system is designed to exhaust hot air into a space for applications such as heating a greenhouse or to dry rice with excess air in the range of 60-120% and a clean (less than 10 ppm NOx) flue gas. Krill's system is not suited for heating external surfaces as required in numerous applications.
The present invention supplies a high-temperature uniform heat via a cylinder-shaped radial burner. The curved plenum, porous mesh, divider vanes, multiple inlet ports, and extended upstream fuel/air mixing point provide for uniform distribution of a preheated fuel/air mixture which avoids pre-ignition by limiting the residence time in the plenum to less than 100 msec.